1. Field of the Invention
The invention generally relates to electrochemical cells for use in powering implantable devices and in particular to improved binder materials for use in the formation of cathode electrode structures for use in such electrochemical cells.
2. Description of the Related Art
A wide range of implantable electronic devices are provided for surgical implantation into humans or animals. One common example is the cardiac pacemaker. Other examples of implantable devices include devices for stimulating or sensing portions of the brain, spinal cord, muscles, bones, nerves, glands or other body organs or tissues.
Implantable devices are becoming more and more complex and commonly include sophisticated data processing hardware such as microprocessors, memory devices, or other large scale integration (LSI) devices. Often, the devices are designed for transmitting signals to remote sensing devices. With the increase in the sophistication of implantable devices and in particular with the need to reliably transmit signals to sensors external to the body, the need for improved power cells for powering the implantable devices has increased greatly. There are, of course, limitations on the design and configuration of power cells for use in implantable devices, especially with regard to the size and shape thereof. Moreover, the power cells for the implantable devices must be highly reliable and be capable of providing an adequate amount of current and voltage for an extended period of time.
One type of power supply for use in an implantable device is an electrochemical cell. Examples include cells employing lithium as an anode material. Typically, within such cells, a metal foil anode coated with lithium is provided in combination with a cathode structure having a cathode material mounted on a current collector. The cathode material includes an active cathode compound, such as polycarbon monoflouride, bound to carbon by a single binder compound. A polymeric separator is positioned between the anode and the cathode material thereby forming an electrode structure. The electrode structure is mounted within a cell housing which is flooded with a liquid non-aqueous electrolyte. Appropriate electrical contacts are provided to the anode and cathode.
Within such electrochemical cells, the formation of the cathode material is particularly important. As noted, the cathode material includes an active cathode compound, carbon and a binder material. The binder material must adequately bind the active cathode compound both to the carbon of the cathode material and to the current collector on which the cathode material is mounted. Adequate binding must be achieved without unduly limiting or hindering the electrical characteristics of the active cathode compound. The binder must also achieve adequate mechanical coupling of the cathode material to the current collector such that the cathode material does not separate from the current collector during operation of the electrochemical cell. If an inadequate binder material is employed, the cathode material may peel away from the current collector during use of the electrochemical cell. Such is a particular problem for electrochemical cells used within implantable medical devices since the devices are subject to frequent movement after positioning within the human or animal which can dislodge the cathode material. As can be appreciated, any mechanical damage or degradation to the electrode structure could prevent operation of the electrochemical cell and thereby prevent operation of the medical device which, particularly in the case of cardiac pacemakers, may be fatal.
In many electrochemical cells separate binder and carbon compounds are not provided. Rather, high surface area carbon particles are used which not only provide the necessary carbon content for the cathode material but also act as a binder. The high surface area carbon powder is particularly useful within common alkaline batteries using magnesium dioxide, mercuric oxide or silver oxide as active cathode materials. Although effective in alkaline batteries which use a solid electrolyte, high surface area carbon powder is not effective in electrochemical cells employing liquid electrolytes, such as lithium cells, because the binder is dissolved by the liquid electrolyte.
Hence, for electrochemical cells employing a liquid electrolyte, another constraint on the choice of binder materials is that the binder must not degrade in the presence of the electrolyte solution. For electrochemical cells employing a liquid electrolyte, fluoropolymers, either in powder form or in suspensions, are commonly used as the binder compound. An example of a common binder material is a co-polymer, such as Tefzell.RTM., which combines the good bonding properties of otherwise unstable materials with stable materials which alone do not provide adequate bonding. Tefzell.RTM., in particular, combines Teflon.RTM. with stable polymers to provide a stable binding material. Teflon.RTM., by itself, is not effective because metallic lithium within a lithium battery cell degrades the Teflon.RTM..
One problem with cathode structures formed using conventional co-polymer binders is that the cathode structures cannot effectively be employed with foil current collectors because the co-polymers do not adequately adhere to flat metal surfaces. To achieve adequate adherence the cathode material usually must be formed into a slurry then spread onto the thin foil. To form a slurry, the binder materials must be dissolvable in a solvent. Polymers and co-polymers such as Teflon.RTM. and Tefzell.RTM. are not soluble in any solvents and therefore cannot effectively be employed to form a slurry.
Accordingly, use of cathodes employing copolymer binders has been limited to electrochemical cells employing expanded screen current collectors which have rough surfaces formed of a network of interlaced metal strands. The cathode structure is pressed against the expanded screen current collector such that the cathode material becomes intermeshed with the strands of the expanded screen.
Although electrochemical cells employing co-polymer binder materials in combination with expanded screen current collectors have been somewhat successful, many disadvantages remain. One significant disadvantage is that the expanded screen current collectors are more expensive to fabricate than simple foil current collectors. Typically, titanium, 446-stainless steel or other expensive metals are employed. Not only are such metals more expensive, but such metals are quite rigid and the resulting electrode structures therefore cannot easily be formed into desired configurations such as spiral wound configurations or the like. Indeed, when an electrode structure formed of one of such metals is wound into a spiral configuration for mounting within a cylindrical cell housing, damage to the electrode structure often occurs requiring that the electrode structure be discarded. Softer metals which can more effectively be bent into a spiral configuration do not typically have sufficient rigidity to properly hold the cathode material. Moreover, expanded screen current collectors, particularly ones made of soft metals, have considerably greater width than thin foil collectors and thereby consume valuable space within the electrochemical cell, which is a significant problem within electrochemical cells for use in implantable medical devices wherein the size of the cell must be kept to a minimum.
It would be desirable to provide improved binder materials which avoid or overcome the disadvantages described above and it is to that end that aspects of the invention are drawn.